JUDY WOODRUFF: Today marks five years since the armed uprising in Syria began, before it turned into a full-blown civil war. And this grim anniversary was marked with significant diplomatic and military moves.

We start with some background from chief foreign affairs correspondent Margaret Warner.

MARGARET WARNER: A hero's welcome awaited the first Russian pilots to return home from their air campaign in Syria. They flew out of Syria hours after President Vladimir Putin's surprise announcement that the main part of Russia's several thousand forces would withdraw.

PRESIDENT VLADIMIR PUTIN, Russia (through interpreter): I believe that the goal set out to the Ministry of Defense and the armed forces has in large part been fulfilled, and that's why I ordered the minister of defense to start the pullout.

MARGARET WARNER: Moscow has said its six months of intensive bombing was aimed at Islamic State forces. But, by all accounts, the principal targets were Western-backed and other rebels fighting the government of Bashar al-Assad.

In Geneva today, site of talks to resolve the conflict politically, opposition leaders welcomed the Russian decision.

SALIM AL-MUSLAT, Spokesman, High Negotiations Committee: If they are serious about pulling out all these — this will be an end to dictatorship. It will be an end to crimes in Syria, and it will help us to put an end to terrorism there in Syria.

MARGARET WARNER: Syrian officials, meanwhile, insisted Putin's move was made in full coordination with the Assad government.

JUDY WOODRUFF: And Margaret joins me now.

So, Margaret, U.S. officials think this is for real, this pullout, and do they think it's significant?

MARGARET WARNER: Yes, they do, Judy, surprisingly, given all the distrust.

They had no advanced private notice from the Russians about this, but there was an Obama-Putin phone call scheduled for 3:00 in the afternoon here. And they actually do think it's for real.

And what they look at is how it was ballyhooed in Russia. They broke into the dinnertime television hour. He proclaimed the troops were coming home and victory. And one of the headlines in Russia even said “Mission Accomplished,” which is, of course, perhaps rueful.

Secondly, Russia had reached a fork in the road militarily, because their theory had been use airstrikes, and then have Syrian forces take ground. The Syrian forces didn't turn out to be quite competent enough, so Putin was facing a choice: Does Russia put in ground forces? Didn't want to get into that quagmire.

And then, number three, of course, Putin — on the eve of the Syria peace talks, Putin has achieved what he really wanted, which was to be seen as a major player in the Middle East that has to be reckoned with, number two, and, two, to establish that there wouldn't be another leader anywhere in his neighborhood that would be removed by force by Western-backed forces.

JUDY WOODRUFF: So, why do the sources you have been — I know you have been talking to Americans and Russians. Why do they think he did this and why right now?

MARGARET WARNER: They think he did this right now because of the talks beginning right now, and that they wanted to send a message to Assad that, look, we have come in, we have helped you.

As one official said, they're trying to let Assad know, you have got a check, but it's not a blank check. There's a number in it. And you have to get realistic at the talks.

And the foreign minister of Syria just said last week, oh, a red line for us, there's no discussion of any change in government or that Assad might go. And they were just saying, whoa, boy, we're not going to be watching your back that way.

So, I think that was the number one thing. There is no love loss between Putin and Assad. I mean, one of his top officials has been quoted as calling him a bastard and a butcher, so there was — not that Putin acts for sentimental reasons.

But — and then, finally, as I said, he's been recognized, Putin — they think Putin is ready to do this because he's been recognized as a world power and that he will be now more interested in cooperating.

JUDY WOODRUFF: And I was asking because people would say, well, if they're pulling out, why did they go in, in the first place if they thought Assad was a butcher and such a bad actor?

MARGARET WARNER: Well, because it was preserving Russian interests. In other words, there's the naval base on the Mediterranean. They have now been able to also build an air base. They're keeping those, by the way, and they're keeping troops and planes at both.

So, the U.S. is going to be monitoring that closely. And so they prevented a leader from being deposed. And I think that was the number one reason.

JUDY WOODRUFF: Two other quick things, Margaret. Effect on the peace talks?

MARGARET WARNER: Peace talks, Judy, they're not wildly optimistic.

One official said to me, it opens the door more to some kind of agreement. But you have got all those players, as we all know, the U.S. and the Europeans and the Russians and the Iranians and the opposition in the Gulf states.

And it's a real — as somebody said to me, if it were easy, we wouldn't be here five years from the start of the conflict.

JUDY WOODRUFF: And just very quickly, finally, situation on the ground, where does this leave it?

MARGARET WARNER: Basically, Assad is in a stronger position. The question is, can his forces keep and hold the territory that the Russians have cleared open for them?

They believe the opposition was badly pounded by the Russians, so some groups are intact. Some are actually in worse shape. And ISIL, they — the U.S. intelligence believes, has been slightly weakened, has lost a little more territory, but that wasn't said with huge confidence.

So, we are really still on a stalemate on the ground, and then just hoping that all the sides are so tired that they might be ready to deal.